Numerous studies have revealed differences in the transcriptional regulation of B lymphopoiesis in the embryo and the adult. In addition, B lineage cells isolated from fetal liver and adult bone marrow exhibit differential sensitivity to selected cytokines. The central hypothesis of this proposal is that these differences exist because B-1 progenitors and their progeny are the dominant B lineage cells present in the embryo and that the intra- and extra-cellular signals to which they respond are distinct from those that regulate the development of B-2 B cells. This premise is based on recent studies from our laboratory demonstrating that B-1 B cell progenitors, identified by their unusual Lin- CD45Rlow/- CD19+ phenotype, are the major B cell progenitor population present in the fetus. The goals of this proposal are to determine when and where in the embryo B-1 progenitors emerge and test the hypothesis that the regulatory controls operative on them are distinct from those in the B-2 lineage. Aim 1 will take advantage of recent advancements in the phenotypic resolution of B-1 and B-2 progenitors to define when and in which embryonic tissues B-1 and B-2 B specified progenitors appear and expand and test the hypothesis that B-1 B cells are part of the primitive wave of hematopoiesis. Experiments in Aim 2 will use loss and gain of function approaches in order to identify transcription factors whose expression is required for B-1 development and test the hypothesis that the transcriptional regulation of B-1 and B-2 development is distinct. Fetal and adult B lineage cells exhibit distinct responses to various micro environmental and systemic signals, and we propose that these differences are due to the differential effects of these stimuli on B-1 and B-2 progenitors. Testing this possibility is the goal of Aim 3. Taken together, the results form this study will provide a comprehensive picture of fetal B cell development that is of relevance to the fields of immunodeficiency, leukemogenesis, and hematopoietic stem cell transplantation. PUBLIC HEALTH RELEVANCE B cell development initiates during embryogenesis, but little is known about this process. The results obtained from the experiments in this application will provide a comprehensive understanding of fetal B lymphopoiesis that will be of relevance to the fields of immunodeficiency, autoimmunity, leukemogenesis, and hematopoietic stem cell transplantation.